Conventionally, in a rotor of a permanent magnet electric motor, an iron loss (swirl loss and hysteresis loss) is known to be a main loss. When the speed of the rotor is increased, a passing magnetic flux is also increased, thus increasing the iron loss. As a result, the rotor causes a heat. With the rotor so heated as to cause a high temperature, magnetic characteristics of the magnet or magnetic body will be deteriorated. Therefore, cooling of the rotor is needed.
For cooling the above rotor, for example, an “electric rotor” (refer to Patent Literature 1) having a scratching lubricating cooling mechanism where a surface of the rotor contacts a cooling medium and stirring of the cooling medium cools the rotor and stator is provided. Moreover, a method of providing an external oil pump for sending the cooling medium by means of an external pump or other method is provided.
[Patent Literature 1] Japanese Patent Application Laid-Open No. 2001-37129